1. The Field of the Invention
The present invention relates to systems, methods, and apparatus adapted to increase the efficiency of a thermodynamic cycle. In particular, the present invention relates to monitoring and adjusting various parameters of a Kalina Cycle to increase the overall efficiency of the cycle.
2. Background and Relevant Art
Some conventional energy conversion systems allow heat that would otherwise be wasted to be turned into useful energy. One example of an energy conversion system is one that converts thermal energy from a geothermal hot water or industrial waste heat source into electricity. Such thermodynamic system can include Kalina Cycles. A Kalina Cycle is a “closed-loop” thermodynamic cycle used in converting thermal energy to mechanical power by way of a turbine. As with similar “closed-loop” thermodynamic cycles, the Kalina Cycle's efficiency is at least partially dependent on temperatures of the heat source and the cooling source.
Turbines typically cannot directly use the “heat source” and “cooling source;” therefore, a medium, referred to as a “working fluid,” is used to go between the heat source and the cooling source. For example, the heat from relatively hot liquids in a geothermal vent (e.g., “brine”) can be used to heat the working fluid, using one or more heat exchangers. The fluid is heated from a low energy and low temperature fluid state into a relatively high-pressure vapor. The high-pressure vapor, or working stream, can then be passed through one or more turbines, causing the one or more turbines to spin and generate electricity.
In the process of driving the turbine, the vapor expands and exits the turbine at a lower pressure and temperature. After exiting the turbine, the fluid is condensed to a liquid in a condenser using a “cooling source.” A higher cycle efficiency (and thus more power output) can be realized when the pressure differential between the turbine inlet and turbine exhaust is optimized. These pressures are dependent on the “heat source” and “cooling source” temperatures.
When the “heat sources” and “cooling sources” cannot be used directly by a turbine, then the next best thing (for maximizing efficiency) is to have a working fluid that can duplicate these heat and cooling sources as closely as possible. Most non Kalina Cycle “closed-loop” thermodynamic cycles utilize a working fluid that is a single (or pure) component fluid. For example, much of electrical power today is generated by Rankine Cycle based power plants. These plants use pure “water” as the working fluid. Pure working fluids, like water, are typically limited in duplicating the heat and cooling sources. This is because pure fluids boil and condense at a constant temperature. This constant temperature can be in direct conflict with the variable temperature nature of most “heat” and “cooling” sources. The constant versus variable temperature difference between the working fluid and heat/cooling sources is a thermodynamic structural difference that can result in efficiency losses in Rankine Cycle power plants.
Kalina Cycle plants differ from Rankine Cycle plants in at least one very distinctive way. The working fluid in Kalina Cycle plants is typically an ammonia-water mixture. Ammonia-water mixtures have many basic features unlike that of either pure water or pure ammonia. A mixture of the two fluids can perform like a totally new fluid. The essence of the Kalina Cycle takes advantage of the ability of an ammonia-water mixture to boil and condense at a variable temperature—similar to the heat and cooling sources, and thus, better duplicate these sources. This can result in higher cycle efficiency.
Typically when implementing a Kalina Cycle, the temperatures of the heating and cooling sources are determined. Based on this determination, the optimal concentration of the ammonia-water working fluid is calculated to allow the working fluid to best duplicate the heating and cooling sources, and thus, maximize the efficiency of the system.
In addition to the concentration of the ammonia-water working fluid, various other parameters of the Kalina Cycle can influence the overall efficiency of the cycle. Some such parameters include the pressure of the working fluid, and the flow rate of the working fluid in relation to the flow rate of the heating and or cooling source. Typically, each of these parameters is optimized based on an initial determination of the heating and cooling source temperatures and other system parameters. Once these various parameters are initially set, some are rarely adjusted.
One will appreciate, however, that the heating and cooling sources may undergo change both slowly over time and in some cases rapidly. These changes in one or more of the heating and cooling sources can influence the efficiency of the Kalina Cycle. Furthermore, the reduction in efficiency due to these temperatures swing is especially pronounced in applications where the difference between the heat source temperature and the cooling source temperature is low, e.g. low temperature geothermal applications.